This has worked for me for years. Recently something changed with youtube and it no longer automatically makes videos HD.

Good!
But block the YouTube Iframe when injected in a website

Works great but runs all the time, not limited to youtube, it has access to all websites.

Works well at the time of writing, and hopefully it'll stay working. One problem remains for me, I sync Firefox between several PCs with different screens, when I choose 360p as default resolution for my old netbook it changes the default resolution everywhere to 360p. I would like to be able to choose independant default resolutions on each PC to suit each one's screen and processing power. I hope that's possible.

Work great most of the time, but doesn't have any way to deal with videos that don't have the resolution level you have it set to.